Effects of the protein phosphorylation inhibitor genistein on maturation of pig oocytes in vitro.

In vitro maturation of cumulus enclosed and denuded pig oocytes was reversibly inhibited by the protein kinase inhibitor genistein. The half-maximal effect on maturation was observed at 40 micrograms ml-1. Genistein inhibited total protein phosphorylation and synthesis with the same dose-response relationship (ED50: 40 micrograms ml-1). Protein phosphorylation and synthesis patterns were changed by effective concentrations of genistein. Pig oocytes were sensitive to genistein during the first 12 h of in vitro maturation. This genistein sensitive period corresponds closely with the period of sensitivity to the protein synthesis inhibitor cycloheximide. Whereas the inhibition of protein synthesis affects only nuclear membrane breakdown and not chromatin condensation, genistein inhibits both events. The results of these experiments suggest that protein phosphorylation and synthesis play major roles during pig oocyte maturation in vitro. It is concluded that genistein inhibited protein phosphorylation is a regulator of chromatin condensation, whereas both new protein synthesis and phosphorylation appear to be required for nuclear membrane disassembly. Caution about this second conclusion is, however, necessary because of the dual action of genistein on both protein phosphorylation and indirectly on protein synthesis.